Their study of nearly 600 middle schoolers in New Haven, Conn., found that such efforts limited increases in kids' body mass index (BMI -- an estimate of body fat based on height and weight).

The efforts included nutrition newsletters for students and families; making sure school-based meals met federal nutrition guidelines; limiting sugary drinks and encouraging water consumption; and limiting use of food or drink as rewards for good grades and behavior.

By the end of the five-year study, the average BMI increase was 1 percent among kids in schools with nutritional programs and policies, compared with 3 percent to 4 percent elsewhere.

The study was recently published in the American Journal of Preventive Medicine.

"These findings can guide future school and community interventions. Childhood obesity is a serious health threat, and schools are a vital way to reach children and their families to reduce risks and promote health," said lead author Jeannette Ickovics.

Ickovics is a professor of social and behavioral sciences at Yale University.

"These findings strongly support previous administration policies that provided healthier food for all children in public schools," she added in a university news release.

Those policies were recently rolled back by the Trump administration.

"This is some of the strongest evidence we have to date that nutrition education and promoting healthy eating behaviors in the classroom and cafeteria can have a meaningful impact on children's health," said study senior author Marlene Schwartz.

She is director of the Rudd Center for Food Policy and Obesity at the University of Connecticut in Storrs.

"These findings can inform how we approach federal wellness policy requirements and implementation in schools to help mitigate childhood obesity," Schwartz said in the news release.

More than 1 in 5 American teenagers are obese, and as many as half are overweight or obese, according to the researchers.